Controlling reflux in a distillation process



May 16, 19%? M. F. POTTS 3,320,159

CONTROLLING REFLUX IN A DISTILLATION PROCESS Filed June 8, 1964 FRACTIONATOR\ GASOLINE STEAM {$0 E 37 1 k sTRlPPER 29 STEAM l HA 6 -27 I7 24 JSTRIPPER s9 |8- KEROSENE [STEAM 3 F RNACE FEED '0 r STRIPPER H HEATING 1 DISTILLATE STEAM] n INVENTOR. M. F: POTTS BY M ATTORNEYS United States Patent 3,320,159 CONTROLLING REFLUX IN A DISTILLATHON PROCESS Mack F. Potts, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Filed June 8, 1964, Ser. No. 373,398 2 Claims. (Cl. 208-463) This invention relates to a distillation process and apparatus therefor. In another aspect, this invention relates to a distillation process and apparatus therefor wherein an overhead vapor withdrawn from the distillation zone is partially condensed, a portion of the condensed vapor is recycled to the distillation zone as reflux, and the remainder of the overhead vapor in combination with the remainder of the condensed overhead vapor is passed to a second condensing zone wherein condensation of the overhead vapor is completed.

In a conventional distillation process, a feed is passed to a distillation column wherein a portion of the feed is vaporized and withdrawn from the top of the distillation column. The withdrawn vaporized fraction is condensed and a portion of the condensed overhead fraction is recycled to the distillation column as reflux. The cooling energy required to condense the overhead fraction is high and is a substantial portion of the distillation process cost. Therefore, a substantial reduction in the cost of the cooling energy required to condense the overhead fraction would result in a substantial reduction in distillation process costs.

Accordingly, an object of my invention is to provide an improved distillation process and apparatus therefor.

Another object of my invention is to provide a distillation process and apparatus therefor wherein the cooling energy required to condense an overhead vaporous fraction is minimized.

Other objects, advantages and features of my invention will be readily apparent to those skilled in the art from the following description, the drawing and appended claims.

By my invention, I have provided a distillation process and apparatus therefor wherein an overhead product vaporous stream is only partially condensed in a first condensation stage, at least a portion of the resultant condensed fraction is recycled to the distillation zone as reflux, and the remainder of the condensed portion in combination with the remainder of the overhead product vaporous fraction is passed to a second condensation stage wherein condensation of the overhead product vaporous fraction is completed at a lower temperature than the first condensation stage.

My invention results in a substantial saving in distillation process costs. For example, assume that the temperature of a multi-component vaporous fraction withdrawn from a distillation zone requiring reflux must be cooled to a temperature t to effect substantial condensation of the vaporous fraction. Let it further be assumed that by cooling the vaporous fraction to a higher temperature t that suflicient condensed liquid is obtained at a proper temperature to reflux the distillation zone. Separating the refluxed portion from the remainder of the vaporous fraction prior to completing the condensation of the vaporous fraction by cooling to the lower temperature t results in a savings in cooling energy equivalent to the cooling energy required to cool the reflux portion from t2 l0 [1.

An additional advantage of the invention is that by cooling the vaporous fraction to the substantially higher temperature t only the heavier components of the vaporous fraction are recycled to the distillation zone as reflux. This results in a substantial increase in the absorption effect of the reflux liquid, thereby substantially increasing the effectiveness of the reflux stream and the over-all effectiveness of the distillation process.

The drawing is a schematic representation of one embodiment of the invention.

The invention is broadly applicable to distillation processes generally wherein a vaporous multi-component fraction is withdrawn from the distillation zone and a condensed portion of the vaporous multi-component fraction is recycled to the distillation zone as reflux. Although not to be limited thereto, the invention will hereinafter be described as applied to the distillation of a hydrocarbon distillate feed.

Referring to the drawing and using a specific example to illustrate the process, a hydrocarbon distillate fraction having an API gravity of 45.9 is passed at the rate of 28,122 barrels per stream day (b.p.s.d.) via conduit means 10 to a furnace 11. The distillate feed is combined with a recycle stream, hereinafter described, passed to conduit means 10 via conduit means 12, said recycle stream having an API gravity of 37.4, and passed to conduit means 10 at the rate of 1,000 b.p.s.d. The pressure of the combined mixture passed to furnace 11 is p.s.i.g. Within furnace 11, the feedmixture is vaporized and an eflluent vaporous mixture at a temperature of 650 F. and a pressure of 30 p.s.i.g. is passed from furnace 11 via conduit means 13 to fractionator 14.

Fractionator 14 can comprise a 10.5-foot diameter upper section 22 feet in length, a 12.5-foot diameter intermediate section 58 feet in length, and a 4-foot diameter bottoms stripper lower section 18 feet in length. As illustrated, the feed to fractionator 14 is introduced into fractionator 14 below the 6th tray. Fractionator 14 can contain perforated trays, bubble cap trays, plate trays, or other means for effecting intimate contact between countercurrent flowing liquid and vaporous streams. As illustrated, fractionator 14 contains 40 perforated tray members. Steam at 250 p.s.i.g. is introduced into fractionator 14 via conduit means 16 at the rate of 3,150 pounds per hour. Fractionator 14 is operated at a bottom temperature of 550 F., a top temperature of 353 F. and a top pressure of 21 p.s.i.g.

A side reflux stream at a temperature of 528 F. is withdrawn from the 27th tray via conduit means 17, cooled to a temperature of 380 F. by means of a conventional heat exchange means 19, and recycled to the 29th tray of fractionator 14 via conduit means 20. A product liquid side stream is withdrawn from tray 18 of fractionator 14 via conduit means 21 and passed to a stripper 22. Within stripper 22, the side stream is contacted with 250 p.s.i.g. steam passed to stripper 22 via conduit means 23 at the rate of 2,240 pounds per hour. A vaporous stream is withdrawn from stripper 22 via conduit means 24 and recycled to fractionator 14. Heating distillate having an A.P.I. gravity of 39.1 is withdrawn from stripper 22 via conduit means 26 at the rate of 6,384 b.p.s.d.

A second product liquid side stream is withdrawn from the 30th tray of fractionator 14 via conduit means 28 and passed to a stripper 25. Within stripper 25, the side stream is contacted with 250 p.s.i.g. steam passed to stripper 25 via conduit means 31 at the rate of 8,800 pounds per hour. A vaporous stream is withdrawn from stripper 25 via conduit means 32 and recycled to fractionator 14. A kerosene product stream having an API. gravity of 45.2 is withdrawn from stripper 25 via conduit means 33 at the rate of 16,703 b.p.s.d.

A third product liquid side stream is withdrawn from the 35th tray of fractionator 14 via conduit means 34 and passed to a stripper 36. Within stripper 36, the product side stream is contacted with 250 p.s.i.g. steam passed to stripper 36 via conduit means 37 at the rate of 1,500 pounds per hour. A vaporous stream is withdrawn from stripper 36 via conduit means 38 and recycled to fractionator 14. A naphtha product stream having an A.P.I. gravity of 52.1 is withdrawn from stripper 36 via conduit means 39 at the rate of 2,839 b.p.s.d.

A vaporous overhead fraction is withdrawn from fractionator 14 via conduit means 41 and passed to a conventional means for partially condensing the overhead fraction such as an air fin cooler 42. A vapor-liquid effluent mixture is passed from air fin cooler 42 via conduit means 43 to an accumulator 44. A temperature of 170 F. and a pressure of 18 p.s.i.g. is maintained within accumulator 44. The temperature of the effluent mixture withdrawn from air fin cooler 42 is controlled by a conventional temperature controller 46 which transmits a signal to the fan (and controls the pitch of the blades, motor speed, or position of louvers) of air fin cooler 42 responsive to the temperature measurement and a set point representative of the desired efl'luent temperature.

Condensed water is withdrawn from the water leg of accumulator 44 via conduit means 47. A liquid reflux stream is withdrawn from accumulator 44 via conduit means 48 and passed to the 40th tray of fractionator 14 as reflux. The rate of flow of reflux liquid through conduit 48 is controlled by a conventional flow controller 49 further opening and closing valve 50 responsive to a flow measurement in conduit 48.

The remainder of the condensed hydrocarbon liquid Within accumulator 44 is withdrawn from accumulator 44 via conduit means 48 and combined with the vaporous stream withdrawn from accumulator 44 via conduit means 51. That portion of the hydrocarbon liquid combined with the vaporous stream in conduit 51 is passed from accumulator 44 via conduit means 48 and conduit means 52 to conduit 51. The rate of flow of liquid through conduit means 52 is controlled responsive to a liquid level measurement within accumulator 44. Responsive to the liquid level measurement, valve 53 is further opened or closed, thereby manipulating the rate of flow of hydrocarbon liquid through conduit 52 to conduit 51. The combined vapor-liquid mixture in conduit means 51 is passed to a conventional heat exchange means 3'4 wherein the mixture is cooled to a temperature of The cooled vapor 51 and the cooled liquid 52 (higher temperature level) from the first stage cooling (liquid and vapor being in equilibrium) are recombined and are charged to heat exchanger (condenser) 54 for second stage cooling of the admixture to a lower temperature level. By adding the liquid back to the vapor at this cus, we effect a rapid movement of the body of the flowing fluid over the solid cooling surfaces in the exchange 54, which greatly reduces the boundary resistance of the film between the fluid and the solid heat exchange surface by reducing the thickness of the insulating film (thermal resistance) of the fluid, and thereby increases the rate of heat exchange (cooling) for a given capacity of heat exchange apparatus (condenser).

An efliuent vapor-liquid mixture is passed from heat exchange means 54 via conduit means 56 to accumulator 57. Condensed water is withdrawn from the water leg of accumulator 57 via conduit means 58. Accumulator 57 is maintained at 100 F. and at a pressure of p.s.i.g. Uncondensed vapors are withdrawn from accumulator 57 via conduit means 59 (on accumulator 57 pressure control, not shown). A gasoline product fraction is withdrawn from accumulator 57 (on accumulator 57 level control, not shown) via conduit means 60, said gasoline product fraction having an A.P.I. gravity of 64.4 and is withdrawn from accumulator 57 at the rate of 540 b.p.s.d.

Generally, although not to be limited thereto, the vapor-liquid mixture passed to the second condensation zone (heat exchange means 54) is cooled in the second condensation zone sufliciently to condense all the constituents of the said vapor-liquid feed mixture that are normally liquid and desired in this gasoline fraction.

Strippers 22, 25, and 36 are combination steam strippers and vacuum strippers, the products being first steam stripped and then vacuum stripped as shown.

A residual fraction is withdrawn from fractionator 14 via conduit means 61. The residual fraction withdrawn from fractionator 61 has an A.P.I. gravity of 37.4 and is passed (on level control, not shown) to storage at the rate of 1,116 b.p.s.d. As previously noted, a portion of the residue fraction (1,000 b.p.s.d.) is recycled at 550 F. via conduit means 12 to feed conduit 10. The rate of flow of the recycle residue fraction is controlled by a conventional flow controller 62 further opening and closing valve 63 responsive to a rate of flow measurement in conduit 12 and a set point representative of the desired recycled flow rate.

As will be evident to those skilled in the art, various modifications of this invention can be made, or followed, in the light of the foregoing disclosure, without departing from the spirit or scope thereof.

I claim:

1. A method of controlling the reflux to a fractionation zone of the heavier constituents in a vaporous waterhydrocarbon stream from said fractionation zone comprising: (a) cooling said vaporous water-hydrocarbon to a first temperature so that water and at least a portion of the heavier hydrocarbons in said vaporous stream are condensed to thereby form a first liquid vapor stream; b) passing said first liquid-vapor stream to a first accumulation zone being maintained at a temperature at or below said first temperature; (c) withdrawing condensed water from said first accumulation zone; ((1) passing a stream of the condensed heavier hydrocarbons from said first accumulation zone to the top of said fractionation zone as reflux at a predetermined flow rate; (e) removing the vapor from said first accumulation zone; (f) removing a second stream of condensed heavier hydrocarbons from said first accumulation zone in response to liquid level control therein and combining said second stream of condensed heavier hydrocarbons as a liquid with said vapor from said first accumulation zone to form a sec- "ond liquid vapor stream which stream is at a temperature no higher than said first temperature; (g) cooling said second liquid vapor stream to a second temperature lower than said first temperature so that at least a portion of the vapor in said second liquid vapor stream is condensed to form a third liquid vapor stream; (h) passing said third liquid vapor stream to a second accumulation zone; (i) withdrawing liquid hydrocarbon product from said second accumulation zone; (j) withdrawing condensed water from said second accumulation zone; (k) withdrawing vapor from said second accumulation zone.

2. A method of claim 1 wherein said liquid product stream withdrawn from said second accumulation zone is a liquid gasoline product fraction.

References Cited by the Examiner UNITED STATES PATENTS 2,553,469 5/1951 Pellettere 6228 3,024,171 3/1962 Bone 202 3,158,556 11/1964 Hopper 202160 DELBERT E. GANTZ, Primary Examiner.

H. LEVINE, S. P. JONES, Assistant Examiners. 

1. A METHOD OF CONTROLLING THE REFLUX TO A FRACTIONATION ZONE OF THE HEAVIER CONSTITUENTS IN A VOPOROUS WATERHYDROCARBON STREAM FROM SAID FRACTIONATION ZONE COMPRISING: (A) COOLING SAID VAPOROUS WATER-HYDROCARBON TO A FIRST TEMPERATURE SO THAT WATER AND AT LEAST A PORTION OF THE HEAVIER HYDROCARBONS IN SAID VAPOROUS STREAM ARE CONDENSED TO THEREBY FORM A FIRST LIQUID VAPOR STREAM; (B) PASSING SAID FIRST LIQUID-VAPOR STREAM TO A FIRST ACCUMULATION ZONE BEING MAINTAINED AT A TEMPERATURE AT OR BELOW SAID FIRST TEMPERATURE; (C) WITHDRAWING CONDENSED WATER FROM SAID FIRST ACCUMULATION ZONE; (D) PASSING A STREAM OF THE CONDENSED HEAVIER HYDROCARBONS FROM SAID FIRST ACCUMULATION ZONE TO THE TOP OF SAID FRACTIONATION ZONE AS REFLUX AT A PREDETERMINED FLOW RATE; (E) REMOVING THE VAPOR FROM SAID FIRST ACCUMULATION ZONE; (F) REMOVING A SECOND STREAM OF CONDENSED HEAVIER HYDROCARBONS FROM SAID FIRST ACCUMULATION ZONE IN RESPONSE TO LIQUID LEVEL CONTROL THEREIN AND COMBINING SAID SECOND STREAM OF CONDENSED HEAVIER HYDROCARBONS AS A LIQUID WITH SAID VAPOR FROM SAID FIRST ACCUMULATION ZONE TO FORM A SECOND LIQUID VAPOR STREAM WHICH STREAM IS AT A TEMPERTURE NO HIGHER THAN SAID FIRST TEMPERATURE; (G) COOLING SAID SECOND LIQUID VAPOR STREAM TO A SECOND TEMPERATURE LOWER THAN SAID FIRST TEMPERATURE SO THAT AT LEAST A PORTION OF THE VAPOR IN SAID SECOND LIQUID VAPOR STREAM IS CONDENSED TO FORM A THIRD LIQUID VAPOR STREAM; (H) PASSING SAID THIRD LIQUID VAPOR STREAM TO A SECOND ACCUMULATION ZONE; (I) WITHDRAWING LIQUID HYDROCARBON PRODUCT FROM SAID SECOND ACCULULATION ZONE; (J) WITHDRAWING CONDENSED WATER FROM SAID SECOND ACCUMULATION ZONE; (K) WITHDRAWING VAPOR FROM SAID SECOND ACCUMULATION ZONE. 